Whether caused by a backhoe, an ice storm or a pack of hungry rodents, losing a span or bundle of communication channels such as DS3 telephone channels means losing significant revenues. After the first 1.5 seconds of an outage, there is also a significant risk that the outage may disable one or more local offices in the network due to an excess of carrier group alarms. Even if traffic could be rerouted before repair of the lost span occurred, restoration of a disabled local office can also take a very long time. Although there are in existence approaches for restoring a network after a failure has occurred, such as shown in U.S. Pat. No. 4,956,835, issued Sep. 11, 1990, in the name of Wayne D. Grover, such a restoration process takes much longer than the 1.5 seconds referenced above which can cause disablement of a local office. Such a restoration process as disclosed by Grover may be used as a backup whenever a preneed plan has not been formulated and a break occurs.
The present invention accomplishes the desired end result by preplanning, according to internal algorithms, a restoration connection for each possible link failure. The method assumes that all links in a span will fail simultaneously, and so finds restoration connections for this (worse case) scenario. When a failure is noted, a dedicated communication loop is used to inform all nodes in the network to implement their portion of a plan associated with the link in the failed span. In order for the present concept to work, the system needs to have adequate spare capacity in the spans connecting each of the nodes. Further, the (broadcast) internode communication used in implementing the restoration algorithm needs to be substantially instantaneous, so a broadcast mechanism is required to all nodes in the network.
The present concept solves this problem by separating the operation into two parts of planning and recovery. The planning step is time consuming, and it must be completed before the failure happens. In the planning step, a set of emergency plans are calculated for each possible payload or traffic (active circuit) link failure in the network.
When a given link fails, the cross-connects or nodes on each end of the failed link detect the failure. The two nodes detecting the failure are at that moment denoted as custodial nodes and these nodes transmit an alarm message to every other node in the network. Each of these other nodes retrieves a precomputed recovery plan associated with the failed link and alternate paths are connected throughout the network according to these precomputed plans. Traffic is rerouted onto the alternate paths, and the recovery is complete. Although not a part of the present invention, the concept can be extended so that preplanning instantaneously commences to prepare for the possibility of a further break before the first break is repaired since fixing a "cut" line normally takes considerable time. If a further failure occurs before the replanning is completed or if further planning is not implemented until the repair is complete, an algorithm similar to that used for the planning process originally, and possibly somewhat like the referenced patent to Grover, may optionally be used to restore some of the circuits in the further failed span if there is sufficient additional spare capacity to accomplish such a task.